Sterol/lipid interactions are being studied through insoluble film techniques at the air/water interface. Cholesterol is found to create two boundary acyl chains lipid layers through a combination of geometric accomodation and enhanced dispersive interactions and to produce maximum condensation of an expanded host lipid with saturated acyl chains at about 42 mole % cholesterol. Unsaturation or higher temperatures can shift the maximum condensation concentration to about 50 mole %. Above 50 mole % cholesterol is at least metestable in contrast to the behavior of epicholesterol which, because of a tilted conformation, partially or completely segregates. This trend to segregation lowers to ability of epicholesterol to interact with neighboring lipids and is effectively complete at concentrations above about 25 mole % epicholesterol.